Hello David
http://www.postgresql.org/docs/faqs.TODO.html
Consider using hash buckets to do DISTINCT, rather than sorting
This would be beneficial when there are few distinct values. This is
already used by GROUP BY.
Regards
Pavel Stehule
2008/6/3 David Fetter <david(at)fetter(dot)org>:
> Folks,
>
> I've noticed that queries of the form
>
> SELECT DISTNCT foo, bar, baz
> FROM quux
> WHERE ...
>
> perform significantly worse than the equivalent using GROUP BY.
>
> SELECT foo, bar, baz
> FROM quux
> WHERE ...
> GROUP BY foo, bar, baz
>
> Where would I start looking in order to make them actually equivalent
> from the planner's point of view? Also, would back-patching this make
> sense? It doesn't change any APIs, but it does make some queries go
> faster.
>
> Cheers,
> David.
> --
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